The present invention relates to radar signal receivers, in general, and more particularly, to the detection of pulsed radar signals instantaneously over a wide microwave bandwidth, even under the conditions of low false alarm rate requirements the detected pulses especially including those which may be signal levels normally hard to distinguish from the broadband thermal or shot noise, for example, naturally generated by the receiver components.
Some airborne, shipborne or vehicle-borne systems include receivers which are designed to detect pulsed microwave carrier frequency signals in the form of pulse trains or the like that may have been transmitted by one or more radar signal sources. Since the carrier frequency of the transmitting radar is generally not known by the receiving detector and may normally range within a broadband of frequencies, say 2-20 GHz, for example, the detection frequency range of these receivers must be set commensurately wideband in order to detect substantially all such microwave transmissions. However, when pulse detecting across a broadband frequency range in a receiver, there is always the problem of distinguishing electrical signal from electrical noise, like broadband thermal or shot noise, for example, which is naturally generated within the receiver by the various signal amplifying and processing components thereof. That is, the signal level of the receiver noise, while being relatively insignificant at any one frequency, may become comparable to and indistinguishable from the signal level of a weak reception pulse when accumulated over a wideband of frequencies like the broadband detection frequency range or a portion thereof.
Radar signal receivers of the superheterodyne tunable variety have improved the sensitivity of the receiver with regard to weak pulse signal detection in the presence of receiver generated noise by search sweeping across the wideband detection frequency range, approximately 20 GHz, with a narrow tunable frequency band which may be on the order of 10-20 MHz, for example, In these receivers, the noise signal level is accumulated substantially only over the instantaneous bandwidth for any given instant in the pulse detection search sweep, thus reducing the noise signal level by orders of magnitude. Unfortunately, it does require quite a long time to search sweep across the entire detection bandwidth and since this process is one of time sharing, the possibility of radar pulses passing undetected is increased significantly. In other words, the tunable narrow band receivers do not have the characteristics of "instantaneously" broadband detection which may be more than just desirable in some applications.
Another type of radar signal receiver separates the overall broadband frequency detection range into processing channels of predetermined frequency bands on the order of 1-2 GHz each. Each channel processes received signals having carrier frequencies within the channel detection bandwidth. In this arrangement, the detection channels operate concurrently to achieve instantaneous wide bandwidth pulse detection. A typical radar signal receiver of this type may appear as that shown by the schematic block diagram embodiment of FIG. 1.
Referring to FIG. 1, radar energy 10 may be received within the beam of a conventional antenna 12, thereafter amplified by a pre-amplifier 14, and conducted to a frequency band channel separator 6. Within the separator 16, the received signals may be separated according to frequency by a plurality of band pass filters F1, F2, . . . FN and correspondingly amplified by respectively associated RF amplifiers A1, A2, . . . , AN. Signals S1, S2, . . . , SN may exit their corresponding RF amplifiers to be processed by appropriate downstream apparatus of the receiver. In addition, a plurality of microwave couplers C1, C2, . . . , CN may be connected respectively to the outputs of the amplifiers A1, A2, . . . , AN to couple signals representative of S1, S2, . . . , SN to corresponding detection channels CH1, CH2, . . . , CHN.
In a typical channel, such as that shown in CH1, for example, the RF signal coupled thereto may be converted to a signal representative of the video frequency content thereof by a conventional crystal detector 18 and in some cases, the video signal content may be amplified by a video amplifier 20. Thereafter, the video signal may be compared with a predetermined reference signal 22 in a differential amplifier 24, for example, and a logic level effected in accordance with the outcome of the comparison. For example, if the video signal is greater than the reference, a logical one may be rendered by the amplifier 24 over signal line 26; otherwise the signal line 26 may remain at a voltage potential representative of logical zero. The output logic signals, like 26, for example, are indicative of a pulse detection in the frequency range of the detection channel or indicative of a false alarm as a result of the accumulated noise over the frequency band of the channel exceeding the reference level 22.
In order to detect pulses instantaneously across the overall wideband of microwave frequencies, which may be approximately 20 GHz, the logical outputs of the detector channels CH1, CH2, . . . CHN are normally "OR"-ed. This also unfortunately results in an accumulation of the false alarms of all of the channels. If the false alarm rate of the receiver is set at 10 and there are 10 channels, for example, then the reference signal 22 of each channel CHi is generally set high enough so that a false alarm rate of no more than 1 is rendered by each channel. In so doing, the channel's detection sensitivity to received radar pulse information is greatly reduced. Consequently, some of the pulses may pass undetected because of the setting of the reference signal due to false alarm rate requirements.
The simple configuration shown in FIG. 1 is generally adequate for detecting main beam reception pulses; however, it may be of appreciable importance at times to track a pulse train or predict the time of arrival of a next pulse subsequent to the main beam pass of the radar, that is, in the sidelobes where the reception pulses are much weaker. One method for improving the sensitivity of the receiver to detect these weak sidelobe reception pulses propses to multiplex-filter the channel frequency bands into numerous frequency band subchannels, each subchannel with its own crystal detector and differential amplifiers are shown for a typical channel CHi in the functional embodiment depicted in FIG. 2. This, in principal, is expected to give the desired additional improvement in sensitivity, but at a considerable extra cost and size using well known hardware implementation techniques. While each subchannel apparently enjoys a reduction in noise level because of the reduced search bandwidth, the full additional improvement in sensitivity expected may not be completely realized because each channel must allow for false alarms from any subchannel (i.e., "OR"-ring of subchannel output) as has been explained supra.
It is evident, from the above discussions, that an improved sensitivity for an instantaneous broadband pulse detection receiver is needed for weak pulse detection under certain false alarm rate requirements. Additionally, it would be desirable if the instantaneous pulse detection system could achieve the effective noise bandwidth reduction for signal sensitivity without the necessity of a large number of duplicate components in its implementation and without a significant cost penalty.